1. Field of the Invention
This invention generally relates to integrated circuit (IC) thin film processing techniques and, more particularly, to post-crystallization annealing techniques that can be applied to semiconductor or metal films at low temperatures.
2. Description of the Related Art
When forming thin film transistors (TFTs) for use in liquid crystal display (LCD) or other microelectronic circuits, the location of transistors channel regions, the orientation of regular structured polycrystalline silicon (poly-Si) or single-grain-crystalline silicon, and the surface roughness are important issues. This poly-Si material can be used as the active layer of poly-Si TFTs in the fabrication of active-matrix (AM) backplanes. Such backplanes can be used in the fabrication of AM LCDs and can be also combined with other display technologies, such as organic light-emitting diode (OLED) displays.
Poly-Si material is typically formed by the crystallization of initially deposited amorphous Si (a-Si) films. This process can be accomplished via solid-phase-crystallization (SPC), for example, by annealing the a-Si films in a furnace at appropriate temperature and for sufficiently long time. Alternatively, laser annealing can also be used to achieve the phase transformation.
Conventionally, crystallization techniques are applied to a substrate in such a manner as to yield uniform poly-Si film quality throughout the substrate area. In other words, there is no spatial quality differentiation over the area of the substrate. The most important reason for this end result is the inability of conventional methods to achieve such quality differentiation. For example, when a-Si film is annealed in a furnace or by rapid-thermal-annealing, all of the film is exposed to the same temperature, resulting in the same quality of poly-Si material. In the case of conventional laser annealing, some differentiation is possible, but the price, in terms of loss of throughput, is very high for the modest performance gains realized.
Recently, a new laser annealing technique has been developed that allows for significant flexibility in the process techniques, permitting controlled variation in resulting film microstructure. This technique relies on lateral growth of Si grains using very narrow laser beams, that are generated by passing a laser beam through a beam-shaping mask, and projecting the image of the mask to the film that is being annealed. The method is called Laser-Induced Lateral Crystallization (LILaC).
FIG. 1 is a diagram illustrating the LILaC process (prior art). The initially amorphous silicon film is irradiated by a very narrow laser beamlet, with typical widths of a few microns (i.e. 3–5 μm). Such small beamlets are formed by passing the original laser beam through a mask that has open spaces or apertures (see FIG. 2), and projecting the beamlets onto the surface of the annealed Si-film.
FIG. 2 is a conventional laser annealing mask (prior art). Returning to FIG. 1, the sequence of images 1 through 4 illustrates the growth of long silicon grains. A step-and-repeat approach is used. The shaped laser “beamlet” (indicated by the 2 parallel, heavy black lines) irradiates the film and then steps by a distance smaller than half of the width of the slit. As a result of this deliberate advancement of each beamlet, grains are allowed to grow laterally from the crystal seeds of the poly-Si material formed in the previous step. This is equivalent to laterally “pulling” the crystals, as in zone-melting-crystallization (ZMR) method or other similar processes. As a result, the crystal tends to attain very high quality along the “pulling” direction, in the direction of the advancing beamlets. This process occurs simultaneously at each slit on the mask, allowing for rapid crystallization of the area covered by the projection of the mask on the substrate. Once this area is crystallized, the substrate moves to a new (unannealed) location and the process is repeated.
FIG. 3 is a pictorial representation of a system to accomplish the optical projection and the step-and repeat process (prior art). The LILaC process has the potential for creating intentional spatial variations in the quality of the poly-Si material. Such intentional variations can be beneficial for applications where multiple components are integrated on a display, where each component has different specifications and material performance requirements.
Some poly-Si materials formed through the LILaC process have a highly periodical microstructure, where crystal bands of specific width are separated by high-angle grain boundaries. Within the crystal bands, low-angle boundaries are observed with a frequency of occurrence dependent upon certain specifics of the crystallization process, such as film thickness, laser fluence (energy density), pulse duration, and the like. TFTs fabricated on such poly-Si films demonstrate very good characteristics, as long as the direction of conduction is parallel to the direction of the in-crystal low-angle boundaries.
TFTs with greater electron mobility can be fabricated if the substrate crystallization characteristics can be made more isotropic. In other words, the TFT performance depends upon the angle between the main crystalline growth direction, the direction parallel to the laser scanning axis, and the TFT channel. This is due to the formation of sub-boundaries within the crystal domains. The surface roughness at the “hard” grain boundaries, at the edges of the crystal bands/domains, can be significant. This surface roughness prohibits the reduction of the gate insulator thickness, which is one critical step for scaling down the device geometry for future applications. Further, not all these processes can be location controlled. Therefore, by chance only, depending upon the relative size of the crystal domain and the TFT channel length, certain TFTs will not include grain-boundaries in their active area (channel), whereas other TFTs will include one or more boundaries in their active areas. This kind of non-uniformity is highly detrimental for critical-application TFTs where uniformity of characteristics is more essential than absolute performance.
One embodiment of the SLS/LILaC process involves the use f a large array of narrow slits that simultaneously melt and solidify the i thin film in such a way as to fully crystallize the entire film after two asses that are stitched together. The drawback to such an approach is that due to volume expansion of the Si material during solidification, a large peak appears in the center of each irradiated region with a magnitude approximately equal to that of the film thickness. This peak-to-valley roughness can be detrimental to the characteristics of devices subsequently fabricated on the thin film. One way to eliminate or reduce the magnitude of this surface roughness, which has been described in pending application LASER ANNEALING MASK AND METHOD FOR SMOOTHING AN ANNEALED SURFACE, incorporated herein by reference, is to partially melt the Si thin film and cause the mass to redistribute itself to reduce the surface tension of the liquid material. This pending application describes a mask that uses diffractive optics to create a homogenous beam with a reduced transmission in the energy density at the sample plane. This homogenous beam then “flood” irradiates the sample and induces partial melting of the film.
FIG. 4 is a plan view of a mask for partially melting a thin film (prior art). The figure shows the upper right-hand corner of a mask pattern that can be used for creating the homogenous beam that is projected onto the Si film. The white lines running left-to-right are open slits that allow the laser beam to pass through the mask (i.e., 100% transmission), whereas the dark gaps between the slits block the beam off (i.e., 0% transmission). By manipulating the widths of these lines, the overall percentage of light that is made incident on the sample can be precisely controlled due to diffractive effects in the projection system.
Such a method is also applicable to in-situ reduction of surface roughness during other methods of SLS/LILaC processing, such as directional solidification, as the mask design can be easily incorporated into the reticle being used for the SLS/LILaC processing.
The use of a specific width of lines and gaps allows for an optimal transmission of energy to the thin film being processed. For the case of films with a thickness on the order of 500 to 1,000 Å thick, this optimal transmission is roughly 55% in many circumstances. However, unwanted diffractive effects at the edges of the pattern can lead to complete melting of the Si thin film.
It would be advantageous if the above-described post annealing surface smoothing process could be further refined to increase throughput and to reduce processing times.
It would be advantageous if the unintentional diffraction effects associated with post-annealing smoothing mask patterns could be reduced or eliminated.